Spiral drive fastener with stick fit surface and method of manufacture

ABSTRACT

Interference surfaces are constructed on inner transition surfaces between wings of a fastener having a spiral recess. A portion of the transition surface is tapered to provide the interference. A standard driver for a spiral recess will engage the interference surface to create a “stick fit” engagement. The punch is formed using a wire EDM operation to construct tapered surfaces within the die cavity of a hob. The hob is machined down to obtain the desired depth, which is measured up from a gage spot indicative of the sticking point.

This application claims the benefit of U.S. application Ser. No.10/825,633 filed 15 Apr. 2004, which is incorporated by reference hereinin it's entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This application relates in general to drive systems for threadedfasteners, tools for their manufacture, and drivers for applying torqueto such fasteners. More specifically this application relates tofasteners that employ winged type drivers. In particular fastenershaving a recess that provides spiral driving surfaces.

2. Brief Description of Related Developments

Threaded fasteners commonly used in industrial applications typicallyare driven by power tools at high speeds and under high torque loads.Such conditions present difficult design considerations, particularlywith respect to the drive systems and, more particularly, with threadedfasteners having a driver engageable recess in the fastener head.Ideally, such a drive system should be easily manufactured, both as torecess and head geometry as well as to associated tooling for formingthe fastener head and drivers for engaging the recess. The strength ofthe head of the fastener should not be adversely affected by the recess.The driver should be easily insertable into and easily withdrawn fromthe recess. The driver and recess, when mated, should distribute thestress loads uniformly to avoid formation of highly localized regions ofstress that might result in deformation of the recess, or driver, orboth, leading to premature failure of the drive system.

The drive system should resist cam-out of the driver from the recesswhen the fastener is driven. In many applications, it is very importantthat the fastener must be capable of withstanding several cycles, as inapplications where the fasteners must be removed in order to repair orreplace parts or to remove and replace access panels. The fastener drivesystem ideally should be capable of such repeated cycling, particularlyin environments where the recess may become contaminated, painted,corroded or otherwise adversely affected in use. In such environments,it is essential that the drive system maintain driving engagement whileapplying torque in a removal direction. It may be necessary for thedrive system to be capable of applying even higher levels of torque whenremoving the fastener, as may occur when the fastener is over-torquedduring initial assembly, or where corrosion develops at the interface atthe engaged threads, or if thermal cycling of the assembled componentshas placed increased stress on the fastener. Where one or more of these,and other, characteristics may present competing considerations,compromises of one in favor of another may be made.

A variety of recess and driver configurations are in common use,including a number of cross-recesses, such as those described in U.S.Pat. No. 24,878 (Smith et al.); U.S. Pat. No. 3,237,506 (Muenchinger)and U.S. Pat. No. 2,474,994 (Tomalis). Other fastener geometries includemulti-lobe geometries of the type described in U.S. Pat. No. 3,763,725(Reiland) and ribbed drive systems as described in U.S. Pat. No.4,187,892 (Simmons). Also among the common recess configurations is theAllen system which is essentially a straight walled hexagonally shapedsocket receptive to a similarly shaped driver.

With the exception of the ribbed systems, the walls and faces of thedriver and recess typically are designed to fit closely with each otherin an effort to achieve face-to-face contact of the driving and drivensurfaces. With cross-recess fasteners, such face-to-face engagement canoccur only, if at all, when the driver is properly aligned and seatedwithin the recess. As a practical matter, however, in order to enablethe driver to be inserted into the recess, there necessarily must besome clearance between the two.

The necessity for such clearance is even more critical with recesseshaving substantially vertical drive walls, as in the Reiland '725 patentand Allen head systems. In all of these systems, the practical result ofthe necessity for such clearance is that substantial face-to-face, broadarea contact between the driver and recess surfaces is seldom achieved,if at all. With most drive systems for threaded fasteners, the drivermates with the recess in the head in a manner that results in point orline contact rather than face-to-face broad area contact. The actualarea of contact typically is substantially less than full face-to-facecontact. Consequently, when torque is applied by the driver, the forcesapplied to the screw head tend to be concentrated in localized areaswith resulting high localized stresses. Such localized high stress canplastically deform the recess, forming ramps or other deformationsresulting in premature, unintended disengagement of the driver from therecess. The foregoing difficulties have been recognized in the art. Forexample, U.S. Pat. No. 2,248,695 (Bradshaw) discloses a screw head anddriver arrangement in which the driving and driven faces of the driverand fastener, respectively, are curved and located eccentrically withrespect to the screw axis. In the Bradshaw fastener, any “suitablecurvature” such as circular or log spiral may be used as long as it isoriented to bind or lock together by frictional engagement.Notwithstanding Bradshaw's teachings, later fastener drive systems, asthose referred to above, do not appear to have adopted the Bradshawteaching of relying on frictional engagement.

A drive system for maximizing the engageable surface area between thedriver and recess is described in U.S. Pat. No. 5,957,645, which iscommonly owned with the subject application. The disclosure of the '645patent is incorporated in this application by reference. The recess anddriver of the '645 patent are constructed with spirally configuredengaging surfaces.

The advantages of the invention described in the '645 patent areachieved by configuring the driving and driven surfaces of the driverand fastener, respectively, to conform to a segment of a spiral and,particularly, in a spiral configuration that enables a substantial andgenerous clearance between the driver and the recess during insertionand removal of the driver but in which the fully seated driver ispermitted to rotate to take up that clearance. The spiral configurationsof the drive walls of the driver and the driver-engageable walls of therecess are such that when the spiral walls engage, they do so over arelatively broad area thereby applying and distributing the stress overthat broad area. The spirally configured driving and driven walls areoriented to direct a major portion of the applied torque substantiallynormal to the fastener radius with little, if any, reliance onfrictional, near-tangential engagement.

In many applications in which a spirally configured drive system isused, the driver may be power driven or required to be inserted inlocations of limited access. In such situations, there is a need toreleasably engage the fastener on the driver in advance of installationso that the driver can be used as an insertion tool, as well as adriver. This “stick fit” feature has been attempted in several differenttypes of fasteners, for example, as shown in U.S. Pat. Nos. 6,199,455;4,457,654; and 4,084,478.

In the '478 patent a series of ribs 26 are inserted in the recess toprovide the interference fit. In the '654 patent a cap is installed overthe screw head, pressed into the recess and a detent, annular ridge 30,is provided in the cap to engage the driver. In the modified squaredrive fastener of the '455 patent, adjoining outer walls 17 and 23 aretapered to provide a stick fit feature. The surfaces 17 and 23 areselected for their larger area relative to driving surfaces 14.

The “stick fit” feature allows the fastener to be releasably engaged onthe driver to enable manipulation of the driver and fastener as a unitin hard to reach, automated, and other applications. Once installed, thefastener and driver may be disengaged with minimal effort.

The '645 patent indicates that the a positive draft may be formed in theinstallation, removal, and other surfaces of the recess. Although itisn't clear from the description, such a draft may be for the purpose offacilitating the engagement of the driver in the recess or providing adegree of latitude for the driver orientation other than straight axialalignment. There is no indication that such a draft would be used forthe purpose of providing a “stick fit” feature on this type of fastener.In fact the suggested draft of up to 6 degrees, is far in excess of whatwould be needed and would significantly interfere with the purpose ofthe spiral installation surface, which is namely to provide engagementof the driver and fastener along an extended surface area as opposed tothe tangential engagement of other drive systems. It is therefore neededto provide a suitable means of providing a stick fit feature on aspirally configured fastener.

SUMMARY OF THE INVENTION

It is the purpose of this invention to provide an interference fitbetween the driver and fastener in which the recess and driver surfacesengage along a spirally configured surface. In order to accomplish thispurpose, the recess is dimensioned as close as possible to the standardspirally configured recess to accommodate currently available drivers.The lobes or wings of the recess are formed with an installation surfaceand a removal surface. Adjacent wings are separated at the inner radiusof the lobe by a transition surface.

To generate an interference fit, the transition surface is constructedwith a slight draft of up to 2° in which the distance at the bottom ofthe recess between opposing transition surfaces is slightly reduced. Thestandard driver will be insertable in a normal manner up to apredetermined depth after which the reduced dimension will cause aninterference with the mating transition surfaces on the driver. This isaccomplished, while minimizing any change in dimension across the upperopening of the recess. In this manner a “stick fit” feature is providedwithout appreciably altering the desired full surface area engagement atthe installation surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The coupling of this invention is explained in more detail below withreference to the accompanying drawing, in which:

FIG. 1 is perspective view of a flat head screw having a spirallyconfigured recess;

FIG. 2 is a perspective view of a flat head screw having a spirallyconfigured recess modified according to this invention;

FIG. 3 is a top view of the flat head screw of FIG. 2;

FIG. 4 is a sectional view of the flat head screw of FIG. 2 taken alongsection lines 4-4 of FIG. 3;

FIG. 5 is a perspective view of a die for forming a punch used inconstruction the flat head screw of this invention;

FIG. 6 is a perspective view of a die blank prior to a wire EDMoperation;

FIG. 7 is a schematic view showing gauging of the die;

FIGS. 8 a and 8 b are top and sectional views of the die of FIG. 5; and

FIGS. 9 a and 9 b are top and sectional views of the die of FIG. 5machined to a predetermined depth.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

FIG. 1 illustrates an exemplary threaded fastener 10 having a shank 12with threads 14 formed at one end and a head 16 with a recess 18 formedat the other end. The head 16 may be formed in a conventional two-blowheader machine in which the end of the wire or other material from whichthe fastener is made is supported in a die of the header machine and itshead end is impacted, first with a punch that partially forms the head,and then with a finishing punch that finishes the head and forms thedriver-engageable recess. The recess 18 is illustrated as having acentral portion 20 and a plurality of radially outwardly extending lobes(wings) 22. The recess in the embodiment of FIG. 1 is formed so thateach of its wings 22 has an installation wall 24 (assuming aright-handed thread 14) and a removal wall 26. The installation wall 24and removal wall 26 preferably are formed to be substantially vertical,defining or closely approximating a cylindrical surface parallel to thelongitudinal axis of the screw. The bottom of the recess may be definedby a conical bottom wall 28. The radially outer end of each wing may becontoured to provide a smoothly curved transition 30 between theinstallation wall 24 and removal wall 26 of the wing. The recess alsoincludes an inner transition wall 32 between the installation wall 24 ofone wing 22 and the removal wall 26 of the next adjacent wing 22. Itshould be understood that the invention may be incorporated in threadedfasteners in a wide variety of configurations including recesses havingone or more wings in which only one of the installation or removal walls24, 26 is provided with a spirally contoured segment.

The fastener 20 of this application is shown in FIG. 2 and provides areliable “stick fit” for receiving a standard spiral driver in aninterference fit. “Stick fit is provided by constructing a limitedinterference surface 40 on the transition surface 32. The interferencesurface 40 is formed at a draft angle θ in the range of 1°-2° with theaxis 41 of the fastener, as shown in FIG. 4.

As can be seen in FIG. 3, the opposing interference surfaces 40 willprovide opposing tapered surfaces within recess 20 at the lower portionsof transition surfaces 32. The recess 20 is constructed having asubstantially standard sized opening across the top of the recess of d₁,but a reduced size of d₂ at the bottom. The interference surface 40causes an interference fit with a spiral driver, as the driver is fullyinserted into the recess 20. Since the interference occurs only when thedriver is fully inserted in the recess 20, the “stick fit” feature ofthis invention does not detract from the surface to surface engagementof installation walls 24 and removal walls 26 with the driving surfacesof the driver.

In order to manufacture the tapered surfaces 40 on transition surface32, a wire EDM operation is used to construct a die or hob 50, as shownin FIG. 5, for cold forming a punch from which the recess of thisinvention can be made. The hob 50 is constructed from a blank 51 ofsteel bar stock, for example, one inch long by one inch in diameter,that is centerless ground to a tolerance of one inch +0.0005/−0.0000.The ends of the blank need to be parallel and square to the outsidediameter. Close tolerances are necessary to ensure a successful wire EDMoperation. As shown in FIG. 6, a central bore 52 is drilled axially inthe blank 51 at a diameter less than the d₁ dimension shown in FIG. 3.

The fully dimensioned and drilled blank 51 is then positioned in a wireEDM fixture with the wire inserted through the bore 52. The wire EDM isprogrammed to cut out a block of material to form a die cavity 53 in theshape of the punch for the spiral recess, as shown in FIG. 2. To achievethe “stick fit” configuration, the top opening of the cavity, betweenopposing interference surfaces 40, will be to the dimension d₁ and atthe bottom opening of the cavity this distance will be to the dimensiond₂, where d₁>d₂. This tapered configuration is formed only on thetransition surfaces 32 to generate the interference surfaces 40.

In order to construct the die cavity 53 of hob 50 to the desired depthfor forming a punch (not shown). It is important to locate the stickingpoint at which the sticking occurs. To locate this gage spot, aspecialized gage 54 having a dial 55 and a probe 56 is constructed inwhich the probe 56 is shaped as a driver for a standard spiral fastener10, for example, as shown in FIG. 1. Probe 56 is inserted into cavity 53until interference is indicated on dial 55. This is the depth x of thegage spot and is used as the point from which the depth y of the cavityis determined. The upper face 57 of the hob 50 is then machined toobtain the depth y. This is shown in FIGS. 8 and 9. Once fullydimensioned and machined, hob 50 is hardened and tempered to a hardnesslevel of 60 to 63 on a “C” scale hardness tester. This process permitsthe reliable construction of multiple hobs having the same dimensions.

A punch is then constructed by pressing the hob 50 in a power press ontoa soft steel blank, as is well known in the art.

While the invention has been described with reference to specificembodiments, the description of the specific embodiments is illustrativeonly and is not to be construed as limiting the scope of the invention.Various other modifications and changes may occur to those skilled inthe art without departing from the spirit and scope of the invention, asdefined by the claims.

1. A method of manufacturing a fastener, wherein said fastener isconstructed having a shank with a longitudinal axis, said shankconstructed having a recess formed at its end, the recess having acentral portion and a plurality of wings radiating outwardly from thecentral portion, each of the wings having an installation wall and aremoval wall, the wings being configured so that at least one of theinstallation or removal walls defines a segment of a spiral, said methodcomprising the steps of: constructing a cylindrical steel blank having apredetermined length; drilling an axial bore from a top to a bottom ofsaid blank; inserting a wire through said bore for providing an EDMoperation; machining said recess using said wire EDM operation to form adie cavity; in said machining operation, constructing a transitionsurface connecting said installation and removal walls of adjacentwings, said transition surface extending from said top portion of saidbore to bottom portion of said bore; wherein said transition surface isconstructed having a first radial distance from the longitudinal axis atsaid top portion of said bore to a second radial distance from saidlongitudinal axis at said bottom portion of said bore, and wherein saidfirst radial distance is larger than said second radial distance;determining the level in said recess where said interference fit occurs;measuring a predetermined depth of said recess from said level wheresaid interference occurs; and machining the top of said blank to saidpredetermined depth.
 2. A method of manufacturing a fastener, accordingto claim 6, further comprising the step of: hardening said blank to forma hob having a die cavity for use in forming a punch; forming a punchusing said hob.
 3. A method of manufacturing a fastener, according toclaim 8, further comprising the step of: forming a recess on the shankby using said punch.
 4. A method of manufacturing a fastener, accordingto claim 6, wherein said interference surface is constructed to form anangle with a line parallel to said longitudinal axis in a range ofbetween 0.5 degrees to 2 degrees.
 5. A method of manufacturing afastener, according to claim 6, wherein said recess is formed havingtransition surfaces diametrically opposed across said recess and saidinterference surfaces on said opposing transition surfaces areconstructed to cooperate to form an interference fit with a driverconstructed to engage said recess.